1. Field of the Invention
The present invention relates generally to corrosion testing of steam generator tubes and, more particularly, is concerned with a nondestructive ultrasonic system and method for measuring and imaging the thickness of steam generator tubes to determine corrosion attack depths.
2. Description of the Prior Art
Heat produced by fission in a nuclear reactor core of a nuclear power plant is transferred to a primary reactor coolant flowing through the reactor core. The primary reactor coolant then flows through steam generators of the nuclear power plant where it transfers the heat to a secondary feedwater which is transformed thereby into steam. The steam is used to generate electricity by driving a conventional steam turbine-electrical generator apparatus.
Each steam generator has a large bundle of tubes. The high temperature primary reactor coolant flows through the secondary interior of the tubes in heat exchange relationship with the feedwater flowing along the exterior of the tubes. The primary reactor coolant flowing through the steam generator tubes is a source of corrosion of the walls of the tubes which reduces wall thickness and can eventually lead to wall perforations.
As a result, long term corrosion tests of steam generator tubes are conducted in order to understand the corrosion generating mechanism and to determine typical steam generator tube corrosion rates. Depth of corrosion damage of tube specimens found by these tests is normally 0.003 to 0.005 inch. Several testing methods have been used to measure the corrosion attack depths, both destructive and nondestructive of the tube specimen undergoing inspection.
A destructive testing method employed in the past is surface profilometry to measure corrosion attack depth. This method of damage determination is satisfactory from the standpoint that its minimum sensitivity to attack depth of 0.001 inch is less than the typical corrosion depths encountered of 0.003-0.005 inch. However, it is unsatisfactory because it requires destructive sectioning of the tube in that tube specimens must be cut to size and descaled to remove deposits before profilometry can be performed. Destruction of test specimens means that resumption of long term tests following interim examinations with the same test specimens, which is highly desirable, is not possible. Thus, the rate of damage growth in long term corrosion tests has not been measured, but instead has been inferred by comparing the profilometry results from different specimens which operated for various periods of time. This is considered unsatisfactory since damage rates are not reproducible on all specimens within a test.
Two nondestructive testing methods have been used heretofore to determine attack depths during interim examinations of long term tests. These inspection methods include eddy current testing (ET) and ultrasonic test measuring (UTM). The ET method has been unsatisfactory for laboratory tests because the typical attack depths encountered of 0.003-0.005 inch are less than the maximum sensitivity of this technique of 0.005-0.010 inch, or 10% of the tube wall thickness. Therefore, ET provides useful detection but does not provide useful sizing information concerning the early stages of corrosion. In contrast thereto, the UTM method is capable of determining the normally observed attack depths of 0.003-0.005 inch since its minimum sensitivity is 0.001 inch and with focused beam probes measure an area of approximately 0.010 inch by 0.010 inch.
The UTM method is performed by placing an ultrasonic probe inside the tube. The tube is filled with water to allow the ultrasonic wave to travel from the probe through the water and into the tube wall. The reflected ultrasonic wave is monitored by the same probe. The time required for the ultrasonic wave to complete a round trip in the tube wall is proportional to the tube wall thickness. The relationship between wave travel time and tube thickness is: EQU T=(vt)/2
where:
T=tube wall thickness (inches) PA1 v=velocity of ultrasound in Inconel (0.23 inch/micro-sec, a material constant) PA1 t=travel time (micro-seconds).
Since the nondestructive ultrasonic test method is capable of determining the normally observed attack depths, its use is desirable. However, this inspection method is currently manually operated, and the associated data reduction is also performed manually. Because of the large manpower effort required to perform and evaluate ultrasonic tube inspections, effective use of this desired method is not feasible.
Consequently, a need exists for a way to automate data collection and reduction associated with the UTM method of testing tubes so as to make its use feasible.